Phenolic resin compositions are materials having excellent heat resistance and adhesiveness to inorganic fillers and are widely used as binders for friction materials such as brakes. Specific examples of phenolic resin compositions that are generally used as the above-described binders for friction materials include powder-form thermosetting phenolic resin compositions obtained by grinding and mixing a random novolac phenolic resin and hexamethylenetetramine and the like.
In typical processes of the related art for manufacturing friction materials such as brakes for which a phenolic resin composition is used, a resin mixture obtained by mixing a fibrous inorganic filler base material such as glass fibers, aramid fibers, or metal fibers, a powder-form inorganic filler such as calcium carbonate or barium sulfate, or the like with a thermosetting phenolic resin composition which is used as a binder is heated, pressurized, and molded using a hot press device, thereby producing compacts.
In addition, thus far, there has been a demand for the above-described binders for friction materials to have characteristics such as a high strength, heat resistance, wear resistance, a high coefficient of friction, a low noise, low moisture absorption properties, and high vibration absorption properties. Particularly, in recent years, in order to increase the productivity of friction materials, there has been a demand for improving the curing properties of phenolic resin compositions for friction materials. In order to satisfy the above-described demands, a variety of studies have thus far been performed regarding phenolic resin compositions for friction materials.
For example, Patent Document 1 discloses a technique in which, in order to improve the curing properties of phenolic resin compositions for friction materials, a high-ortho novolac phenolic resin is used as a novolac phenolic resin, an acidic substance is added in order to increase the decomposition rate of hexamethylenetetramine, and a high-ortho novolac phenolic resin and hexamethylenetetramine are made to become an adduct by means of melting and kneading.